1. Field of the Invention
This invention relates to a method of and apparatus for deep drawing metal articles and more particularly to such method and apparatus which reduces the tendency of metal to wrinkle or buckle in the redraw step of a draw and redraw can forming operation.
2. Description of the Prior Art
Deep drawing of can bodies from flat sheet metal, and particularly from sheet steel such as tinplate, has presented problems due to the tendency of the sheet material to buckle or form wrinkles during the drawing process. Such wrinkles may result in unsightly lines or structural defects in the sidewall of the finished can or in metal failure during the drawing operation.
In deep drawing sheet metal into a cylindrical configuration, the peripheral portion of a flat circular blank is clamped between opposed, general planar holding surfaces, and the central portion is pushed through an opening in a die by use of a cylindrical punch. As the punch telescopes through the die, the clamped peripheral portion of the blank is drawn radially inward and placed under very high compressive stresses in the circumferential direction while simultaneously being subjected to substantial tensile loads in the radial direction. The high compressive stresses increase progressively outward of the blank, with the result that there is a tendency for the thickness of the blank to be increased at its peripheral edge portion and thereby separate slightly the clamping surfaces. If the peripheral edge portion is permitted to thicken, the compressive stresses tend to buckle or form wrinkles in the radially inner portion adjacent the inner periphery of the die. These wrinkles not only produce an uneven appearance in the formed sidewall of the drawn container, but also produce stress concentration which can result in tearing of the metal under the heavy tensile loads.
In a shallow drawing operation, the stresses encountered will not exceed the strength of the metal being drawn; however, for deeper drawing operations, particularly when using steel rather than a softer metal such as aluminum, it has generally been necessary to use several drawing steps each producing a cup of progressively smaller diameter without exceeding the stress limits of the material being formed. It has generally been considered essential that all redrawing steps, i.e. drawing steps subsequent to the initial cupping step, progressively reduce the diameter of the cup in relatively small increments. This has been particularly true in deep drawing high strength steel materials where very high stresses are required. However, it is desirable to employ the minimum number of drawing steps both from the standpoint of reducing time and expense of the drawing operation and to reduce the adverse effects on the base material and on any metallic, chemical or organic coatings on such base material.
My prior U.S. Pat. No. 3,494,169 discloses a method of and apparatus for single stroke blanking and deep drawing of flat container sheet metal stock while maintaining a more uniform clamping load during the drawing operation. This is accomplished by utilizing a plurality of independently loaded concentric clamping rings which sequentially contact the flat circular blank of sheet metal to maintain a substantially uniform clamping force on the metal blank. The concentric clamping rings are arranged so that the largest diameter ring contacts the outer peripheral portion of the blank initially, with subsequent clamping rings engaging the blank as its diameter is progressively reduced. As the blank passes from beneath a clamping ring, that ring engages the opposed clamping surface of the movable annular die, thereby continuing to resist the drawing load applied to the blank. Each annular clamping ring applies its load, from a separate biasing means, independently of the load applied by other clamping rings.
U.S. Pat. No. 4,195,510 to Jurgens discloses an apparatus for drawing articles from flat sheet material including a clamping die plate including a draw die bead having alternate ridges and grooves in the clamping area adapted to permit slight thickening of the metal blank. In one embodiment the draw bead is formed in a plurality of parallel section each supported by a resilient pad to permit relative movement in a direction parallel to the drawing direction.
While the prior art method and apparatus disclosed in the above-mentioned patents have been effective, at least to some extend, in increasing the permissible depth of draw in the initial drawing operation from a flat blank, a one-step drawing operation generally cannot be employed in a high-speed can drawing operation to deep draw can bodies of the type employed, for example, as food and beverage containers. Accordingly, it is the primariy object of the present invention to provide an improved method of and apparatus for deep drawing of can bodies from flat sheet metal.
It is another object of the invention to provide an improved method of and apparatus for fomring a deep drawn can body requiring only a single redrawing step.
Another object of the invention is to provide an improved method of and apparatus for deep drawing can bodies in which flat circular blanks are initially drawn into shallow cups in a first drawing operation and in which the drawn cups are transferred to a second drawing station for redrawing on a draw press including a pair of concentric clamping rings adapted to cooperate to provide a more uniform clamping pressure during the redrawing operation.
Another object of the invention is to provide an improved method of and apparatus for redrawing a shallow cylindrical cup into a deep can body having a more uniform wall thickness and improved sidewall appearance.